1. Field of the Invention
The invention is related to a semiconductor wafer used for various devices including electronic devices such as transistors. More specifically, the invention is on a compound semiconductor wafer composed of group III nitride.
2. Description of the Existing Technology
(Note: This patent application refers several publications and patents as indicated with numbers within brackets, e.g., [x]. A list of these publications and patents can be found in the section entitled “References.”)
Gallium nitride (GaN) and its related group III nitride alloys are the key material for various electronic devices such as microwave power transistors and solar-blind photo detectors. However, the majority of these devices are grown epitaxially on heterogeneous substrates (or wafers), such as sapphire and silicon carbide since GaN wafers are extremely expensive compared to these heteroepitaxial substrates. The heteroepitaxial growth of group III nitride causes highly defected or even cracked films, which hinder the realization of high-end electronic devices, such high-power microwave transistors.
To solve all fundamental problems caused by heteroepitaxy, it is indispensable to utilize group III nitride wafers sliced from bulk group III nitride crystal ingots. For the majority of devices, GaN wafers are favorable because it is relatively easy to control the conductivity of the wafer and GaN wafer will provide the smallest lattice/thermal mismatch with device layers. However, due to the high melting point and high nitrogen vapor pressure at elevated temperature, it has been difficult to grow GaN crystal ingots. Currently, majority of commercially available GaN wafers are produced by a method called hydride vapor phase epitaxy (HVPE). HVPE is a vapor phase method, which has a difficulty in reducing dislocation density less than 105 cm−2.
To obtain high-quality GaN wafers of which density of dislocations and/or grain boundaries is less than 105 cm−2, a new method called ammonothermal growth has been developed [1-6]. Recently, high-quality GaN wafers having density of dislocations and/or grain boundaries less than 105 cm−2 can be obtained by the ammonothermal growth. However, a GaN ingot grown by the ammonothermal method typically shows n-type conductivity, which is not favorable to high-electron mobility transistors (HEMT). Due to high-frequency operation, conductive substrate causes high-level of capacitance loss through the substrate. To improve the performance of transistors, semi-insulating wafers are strongly demanded.